The invention relates to semiconductor processing, and more particularly to a bonded silicon-on-insulator wafer structure and method for making the same.
Semiconductor-on-insulator (SOI) technology, particularly silicon-on-insulator technology, has found increased use recently for the improved circuit performance that it provides. In SOI technology, a wafer or other device substrate includes a thin SOI layer consisting essentially of a single crystal semiconductor, e.g., silicon, disposed as a top layer of the wafer. The SOI layer overlies a buried insulating layer, typically of oxide (a “buried oxide” or “BOX” layer), which, in turn, is disposed above a bulk layer of the substrate. Active devices including transistors, diodes and sometimes other devices are formed in the thin SOI layer. The insulating layer eliminates junction capacitance, which ordinarily exists for devices that are formed in a bulk substrate. In such way, devices formed in SOI substrates generally exhibit greater performance than comparable devices provided in bulk substrates.
However, one obstacle to greater acceptance of SOI technology is increased cost of manufacturing SOI wafers on which to form semiconductor devices. Two main technologies exist for fabricating SOI wafers. One is known as a “separation by implantation of oxygen” (SIMOX) process. In a SIMOX process, the buried oxide layer is formed by implanting a relatively thick (100 nm to 300 nm) region below the substrate surface with oxygen ions and thereafter annealing the substrate to convert the implanted region to an oxide. Among drawbacks of a SIMOX process is cost, since the cost of implantation equipment and the amount time per wafer are both significant. SIMOX processing also requires a relatively long duration anneal at high temperature to convert the oxygen-implanted region to an oxide, which also adds to the cost of fabricating SOI wafers by a SIMOX process. The incidence of defects in SIMOX wafers is another obstacle to greater acceptance of SOI wafers fabricated by a SIMOX process.
Another competing technology for making SOI wafers is known as bonding. In bonding methods, a bond wafer having an oxide layer disposed at the surface is bonded to a base wafer with the oxide layer facing the base wafer. The bond wafer is then typically ground or polished down to a desired final thickness, which remains as the SOI layer of the SOI wafer. One particular drawback of bonding methods is difficulty in reducing the thickness of the bond wafer to the desired final thickness. A typical 300 mm diameter wafer has an initial thickness of about 800 μm. On the other hand, the desired final thickness of the SOI layer is about 250 nm or less, and is more often less than 100 nm for current mainstream SOI devices. Thus, almost all of the bond wafer must be removed to complete the bonded SOI wafer. With so much of the SOI wafer to remove, it would be desirable to use a high-speed technique having a high rate of removing material. However, when a high removal rate is used, it may be difficult to avoid accidentally removing too much material and to obtain a high quality SOI layer of uniform thickness at completion.